1. Field Of The Invention
This invention relates to crosslinked polymer gel compositions that may be used as fracturing fluids for oil and gas wells. More specifically, this invention relates to a method of producing a delayed release borate crosslinking agent, wherein the crosslinking agent is a dried borate-crosslinked water-soluble polysaccharide that reequilibrates when placed in a solution of water-soluble polysaccharide.
2. Background and Description Of The Related Art
Polymer gel plugs ("PGP") are often used as temporary well controls. These gel plugs are fluids, designed to form a stiff gel structure when placed in the wellbore. The PGPs can be used under well temperatures as high as 180.degree. C. Depending upon their specific composition, they may be stable for several hours or for several weeks. When operations are complete, the gel plug may be removed by adding an acid or some other substance to break up its structure. In some well control applications, PGPs may be used when mechanical devices would be impractical, unavailable, or uneconomical.
The gelling agent of the polymer gel plug is typically an aqueous colloid of water soluble polymer. Generally, a high molecular weight water soluble polysaccharide is used. When these polysaccharides hydrate, a viscous gel forms. The viscosity of the resulting gel increases with the concentration of the polymer. The viscosity of the gel may also be increased by adding a crosslinking agent. Two varieties of polymer gel plugs are known in the art; those with crosslinking agents and those without. Gel plug compositions lacking crosslinking agents typically employ higher (2-7% by weight) concentrations of polysaccharide. When a crosslinking agent is used, stiff gel structures can be obtained with lower (0.5-2.0% by weight) concentration of polysaccharide.
At lesser concentrations of polysaccharide and/or crosslinking agent, the resulting gel structure will flow. Although such fluid gel structures are ineffective for plugging subterranean formations, they are useful as fracturing fluids. These fracturing fluids are used to break up the structure and increase the permeability of the hydrocarbon-bearing formation. In addition to being less viscous than a gel plug, the gel component of these fracturing fluids is designed to have a much shorter lifetime.
One method of producing a crosslinked gel entails adding the borate crosslinking agent on the fly to a batch or continuously mixed polysaccharide gelling agent. The borate source usually comprises a rapidly soluble, inexpensive, borate compound such as boric acid, borax or Polybor (Black et al., U.S. Pat. No. 3,227,212). One problem with such crosslinking gels is that they form a stiff gel structure very quickly. This in turn produces high pumping pressures and limits the extent of their use.
While the use of a delayed borate source, such as ulexite or colemanite, will substantially lower the pumping pressure the cost of these crosslinking agents is significantly higher (Mondshine, U.S. Pat. Nos. 4,6201,596 and 4,619,776).
Thus, the need exists for a fracturing fluid that inexpensive yet capable of yielding low pumping pressures.